1. Field of the Invention
This invention relates to electrophotographic printing machines using cut sheets, and more particularly to a printing machine for printing both sides of a cut sheet, namely, a duplex printing machine.
2. Description of the Prior Art
One example of a conventional duplex printing machine, a laser printer, will be described with reference to FIG. 1.
In the printing machine of FIG. 1, a photosensitive drum 1 is charged by a charging unit 2, and a latent image is formed on the photo-sensitive drum 1 with a laser beam 3. The latent image is developed with toner 12 by a developing unit 4.
On the other hand, a sheet 6 supplied by a sheet supplying roller 9 is conveyed to an image transferring section of the photo-sensitive drum 5 by a sheet conveying roller 13, where the toner image on the drum is transferred onto the sheet 6 by an image transferring corotron 5. Thereafter, the sheet 6 is discharged by a peeling corotron 11 so that it is peeled off the drum 1. The sheet 6 thus peeled off is conveyed to a fixing unit 8 by an endless conveyor belt 7 so that the image is fixed on the sheet. The toner remaining on the drum 1 is removed by a cleaning unit 10. Thus, one side of the sheet has been printed.
For double sided printing, the sheet is delivered onto a reversing plate 15 by a pair of sheet delivery rollers 14 and is then moved backwardly into the printing machine by a conventional switch-back mechanism, namely, a reversing roller 16 which is rotated by one of the sheet delivery rollers 14. That is, the sheet 6 is moved along a sheet return path 17 and is again fed to the photosensitive drum 1 by the conveying rollers 13, where the other side of the sheet is treated in the same manner as in the first path. The sheet is conveyed to the fixing unit 8 by the endless conveyor belt 7, where the image is fixed. Thereafter, the sheet 6 is delivered out of the printing machine by the sheet delivery rollers 14. In this operation, a sheet path switching gate 18 provided at the sheet inlet side of the reverse plate 15 is swung from the position A to the position B as shown in FIG. 1 to deliver the sheet, both sides of which have been printed, to a stacker 19.
In the above-described method, as is apparent from FIG. 2, it is impossible to swing the gate 18 as long as a sheet 20, only one side of which has been printed, is present on the reversing plate 15. Therefore, in the case where a sheet 21, both sides of which have been printed, is delivered out of the fixing unit 8, as shown in FIG. 3, the gate 18 is swung after the singly printed sheet 20 is moved back into the printing machine by the reversing roller 16, so that the doubly printed sheet 21 is delivered to the stacker 19. For this purpose it is necessary to convey sheets spaced at relatively long time intervals to thereby prevent sheets from crossing each other on the reverse plate 15. However, since the drum 1 is rotated at a constant speed, if the sheets are conveyed at such long time intervals, then the printing speed of the printing machine is greatly decreased.
FIG. 4 shows another example of the conventional duplex printing machine. In the printing machine, a sheet, one side of which has been printed, is delivered to a switch-back mechanism 23 by a switching gate 22 provided immediately after the fixing unit 8. The sheet is moved backwardly by the switch-back mechanism 23 once more into the printing section so that the other side of the sheet is printed. Thereafter, the switching gate 22 is operated to deliver the doubly printed sheet to the stacker 19. The printing machine of FIG. 4 is disadvantageous in that the sheet conveying path is long, and since the number of switching gates is larger by one than that in the printing machine of FIG. 1, the construction is correspondingly intricate.
In general, in a double sided printing operation, the operator cannot observe the sheet during printing, or he can see only one side of the sheet and if, in the case of printer, the sheets printed are not stacked in the order of page numbers, then the operator must later rearrange the sheets in the order of page numbers. In this connection, both of the two methods described above are disadvantageous in that, in printing only one side of each sheet, the sheets printed are stacked on the stacker 19 in the reverse order of page numbers as shown in FIG. 5, in which reference numeral (1) through (5) designates the first through fifth pages, respectively, and reference character a designates the printed surface.